محاليل زيادة الصلابة بالفورمالين .. كتاب التحميض والطبع والتكبير

Third: Formalin hardening solutions:

After fixation, a formalin-based hardening solution can be prepared, such as Gephart's solution No. 354 and Kodak's solution No. (5-SH).

Also, solutions of formalin and sodium carbonate are sometimes prepared to increase the hardness of gelatin prior to display. (such as Kodak SH 5 solution). Due to the effect of sodium carbonate on gelatin swelling, an amount of sodium sulfate is added to the solution to reduce this harmful effect. Likewise, a certain percentage of anti-fog solutions may be added to the solution (as in Kodak 1-SH solution).
The following is a statement of the composition of the previous solutions.

FIXING INSTALLATION PROCESS
It is not expected that the amount of silver salts reduced to metallic silver during the whole exposure period will exceed about 25% of the amount of silver salt deposited in gelatin, while the remainder will remain in its salt state, which makes the sensitive plate or film subject to blackening if exposed to light after a process Show directly. Therefore, it is imperative to remove excess silver salts in order for the image to become stable and not affected by light.

And if it is possible to do the process of removing excess silver salts
(or more accurately dissolved) by means of solutions of several salts such as (cyanides, thiocyanates, sulphites, or concentrated solutions into the flow of potassium bromide), but it is customary to use the compound known commercially as Hypo and chemically as Sodium thiosulfate . Thiosulphate. It is the main compound in the fixing solution, and the process is carried out with it
Dissolution, in three stages, which we describe as follows:

(a) The conversion of tartar salt into a salt that is insoluble in water.

(b) Conversion of the latter compound into a more water-soluble salt.

(h) Conversion of the second compound into a salt that is easily soluble in water and can be easily removed by washing.

These operations are explained as follows:

1- Silver bromide + Sodium thiosulfate = Silver thiosulfate + Sodium bromide.

Silver thio sulphate is a salt that is insoluble in water and its color is white. Therefore, the color of the film at this stage is Milley white, and its color is not stable and is liable to change, so that if the fixation process was stopped either deliberately for experimentation, or by mistake because of
The presence of an adequate amount of hypo) the silver thiosulfate salt will turn into silver sulphide and the color of the silver will change to black. But if the reaction (fixation) continues, the resulting silver thiosulphate reacts with another part
of Hypo (Sodium Thiosulfate) according to the following equation:

2- Silver-Sodium thiosulphate + Sodium thiosulphate = A compound salt, which is Silver-Sodium thiosulphate, a salt with little solubility in water.

But by continuing the reaction with the rest of the amount of hypo, the following third reaction takes place: = Sod. thiosulfate + Silver-Sod. thiosulphate A compound salt is Silver Bisodium thiosulphate, and this last salt is easily soluble in water and washes away, and thus we have finally got rid of the rest of the silver salts.
We conclude from the foregoing that the necessary conditions for the stabilization process are: (a) that the stabilization solution contains an ample amount of hypo salt sufficient to complete these three aforementioned reactions.

(b) The image stabilization process should take sufficient time to complete the three operations. Based on that, it is not permissible for the negative image to come out of the fixative solution even if the film appears transparent. The film may appear as well even though the last process has not been completed. To ensure the result, the film must be kept in the fixation solution even after it appears transparent. The time required to complete the three operations is estimated at about a quarter of an hour in order to obtain a guaranteed image that will remain in good condition.